Auto-focus systems for automated optical inspection (AOI) of wafers, bare PCBs, flat panel displays (FPDs), thin film transistor (TFT) panels and also for imaging solutions for PCB production are known in the art. In these fields, relatively flat objects such as panels are inspected and/or imaged at close range with a typically small depth of field. In some automated optical inspection systems, a field of view of the camera can be as small as several hundreds of microns to several millimeters while the overall size of the panel can reach up to 2×2.5 m. In other fields, flat objects may be inspected with lower magnification and larger depth of field. The auto-focus systems are typically required to maintain accurate working distances while imaging different portions of the panel. Deviations in working distances across the panel can be due to, for example, inherent tolerances in plate thickness, tolerances in flatness of the panel and/or tolerances in the AOI system (or the like) at different locations along the supporting table.
Many known auto-focus systems use a triangulation method to adjust a working distance of a camera. Typically for triangulation, a point, a line or grid pattern is projected on the surface at an oblique angle and a camera is used to capture a specular reflection of a projected beam. Lateral displacement of the point, line or grid is sensed and a triangulation method is used to relate the detected lateral displacement to a deviation of the camera from a defined working distance. The sensed lateral displacement or detected deviation from focus provides input to an actuator for adjusting positioning of an objective lens of the camera. Sensitivity is a function of the angle of oblique illumination and size of the beam spot.
U.S. Pat. No. 5,136,149 entitled “Method of focusing optical head on object body and automatic focusing device for optical inspection system including tilt detection,” the contents of which are incorporated herein by reference, describes a semiconductor wafer that is supported on a movable table mechanism. In order to maintain the surface of the wafer at the focal point of an objective lens and maintain the angle of the wafer perpendicular to the optical axis of the objective lens, a light beam is directed to the wafer. Reflected light is divided into first and second beams. The first light beam is received by a one-dimensional PSD (position sensing device), while the second light beam is received by a two-dimensional PSD. In response to respective outputs of the one-dimensional PSD and the two-dimensional PSD, the movable table mechanism is driven so as to maintain an in-focus state of the wafer and the objective lens even when the wafer is moved for scanning of respective regions on the wafer.
U.S. Pat. No. 5,604,344 entitled “Autofocussing microscope having a pattern imaging system,” the contents of which are incorporated herein by reference, describes an auto-focus mechanism for a microscope including a pattern imaging system, a single image detector and a pattern focus analyzer. The pattern imaging system images at least one pattern onto an object surface through an objective lens of the microscope along a main optical path of the microscope. The image of the pattern is then combined with an image of the object and is reflected along the main optical path towards an image plane of the microscope. Use of a high contrast pattern is disclosed. The image detector detects the reflected image and the pattern focus analyzer determines sharpness of the pattern by analyzing the output of the image detector. The pattern focus analyzer can also indicate, to the apparatus for changing the distance, to move in a direction of increased focus. A direction of focus is determined by imaging two patterns at a distance δ above and below an object plane of a lens of the auto focusing apparatus and comparing focus of both patterns in the reflected image.
U.S. Pat. No. 7,301,133 entitled “Tracking auto focus system,” the contents of which are incorporated herein by reference, describes a tracking auto-focus system that maintains a microscope pointed at a TFT array continuously in focus so as to eliminate the auto-focus time that would otherwise be required. The tracking auto-focus system includes, in part, a microscope Z actuator, a PSD, an analog-to-digital converter (ADC), a signal conditioner, a digital proportional integrating and differentiating (PID) controller, and a digital-to-analog converter. The PSD together with the ADC and signal conditioner continuously monitor and detect the distance between the microscope's objective lens and the target and supply the measured distance to the amplifier. The PID controller together with the DAC stabilizes the distance separating the microscope's objective lens and the target to maintain the best focus.
An article published in Journal of Physics: Conference Series 139 (2008) 012026 entitled “Projection of speckle patterns for 3D sensing,” the contents of which are incorporated herein by reference, describes a use of projected speckle patterns for sensing depths and thicknesses. Different spatially random patterns are generated at different planes. Due to the speckle phenomenon, the patterns obtained at the different heights are highly random and not correlated to each other. The sensing is based on the change of the speckle pattern with propagation and the lack of correlation between speckle patterns recorded at different depths or lateral locations. The principle is used for mapping thickness of transparent media, for depth ranging and for three dimensional mapping of diffuse objects. It is found that the lack of correlation due to the speckle phenomenon will only be achieved when speckle patterns are taken at lateral or axial distances larger than the transverse or axial speckle size.